The present invention relates generally to a non-aqueous electrolyte secondary battery provided with a positive electrode, a negative electrode, and a non-aqueous electrolyte solution, and more particularly, to a non-aqueous electrolyte secondary battery whose cycle performance is improved when lithium-containing titanium oxide is used as a negative electrode material for its negative electrode.
In recent years, as one of new-type secondary batteries having high power and high energy density, a high electromotive-force non-aqueous electrolyte secondary battery using a non-aqueous electrolytic solution as an electrolyte and utilizing oxidation and reduction of lithium has been developed. An example of such a non-aqueous electrolyte secondary battery generally utilized is the one employing a lithium-containing composite cobalt oxide as a positive electrode material for its positive electrode and a carbon as a negative electrode material for its negative electrode and having battery voltage of approximately 4 V.
On the other hand, more recently, in accordance with the lowing of operating voltages of IC circuits, the demand has been growing for a battery whose battery voltage is approximately 2.5 V. Such a battery is now being developed.
As such a battery, there has been proposed, as in JP, 7-335261, A, a non-aqueous electrolyte secondary battery such that a lithiated cobalt oxide is used as a positive electrode material for its positive electrode while Li4/3Ti5/3O4 is used as a negative electrode material for its negative electrode, and the cycle performance thereof is improved by setting the ratio of the positive electrode material and the negative electrode material in a proper range.
Unfortunately, however, in the battery disclosed in JP, 7-335261, A, a disadvantage exists that a lithiated cobalt oxide is very expensive. Furthermore, the battery is liable to be overdischarged when a charge/discharge process is performed, whereby the cycle performance is degraded.
The inventors of the present invention have thus examined using a lithium-containing nickel oxide, which is less expensive than a lithium-containing cobalt oxide, as a positive electrode material along with using a lithium-containing titanium oxide as a negative electrode material in a non-aqueous electrolyte secondary battery whose operating voltage is approximately 2.5 V.
However, the inventors of the present invention have discovered some problems in using a lithium-containing nickel oxide as a positive electrode material. For example, charging/discharging efficiency is degraded, and when a charge/discharge process is performed in a case where a lithium-containing titanium oxide is used as a negative electrode material, the battery is liable to be overdischarged, whereby the cycle performance is degraded as in the case of the above-mentioned battery using a lithium-containing cobalt oxide.
An object of the present invention is to solve the above-mentioned problems in a non-aqueous electrolyte secondary battery provided with a positive electrode, a negative electrode, and a non-aqueous electrolyte solution. Specifically, an object of the present invention is to provide a non-aqueous electrolyte secondary battery which is excellent in cycle performance by preventing overdischarge in a case where a lithium-containing titanium oxide is used as a negative electrode material for its negative electrode.
A first non-aqueous electrolyte secondary battery according to the present invention is a non-aqueous electrolyte secondary battery provided with a positive electrode, a negative electrode, and a non-aqueous electrolyte solution, wherein a lithium-containing composite nickel oxide is used as a chief component of the positive electrode material for the positive electrode, a lithium-containing titanium oxide is used as a chief component of the negative electrode material for the negative electrode, and the solvent of the non-aqueous electrolyte solution contains a cyclic carbonic ester and a chain carbonic ester, the cyclic carbonic ester and chain carbonic ester being contained in amounts of not less than 10% by volume of the whole solvent, respectively, and the total content of the cyclic carbonic ester and the chain carbonic ester being not less than 60% by volume of the whole solvent.
As in the first non-aqueous electrolyte secondary battery according to the present invention, if the solvent contains a cyclic carbonic ester and a chain carbonic ester in amounts of not less than 10% by volume of the whole solvent, respectively, and the total content of the cyclic carbonic ester and the chain carbonic ester is not less than 60% by volume of the whole solvent in the non-aqueous electrolyte secondary battery using a lithium-containing composite nickel oxide as a chief component of the positive electrode material for the positive electrode and a lithium-containing titanium oxide as a chief component of the negative electrode material for the negative electrode, the side reaction that decrease the battery capacity is prevented, whereby cycle performance of the non-aqueous electrolyte secondary battery is improved.
In the first non-aqueous electrolyte secondary battery, the cyclic carbonic ester and the chain carbonic ester are respectively contained in amounts of not less than 10% by volume of the whole solvent because when the amount of the cyclic carbonic ester is less than that, ionic conductivity in the non-aqueous electrolyte solution is reduced, whereby cycle performance is degraded, and when the chain carbonic ester is less than that, the viscosity of the non-aqueous electrolyte solution is made high, whereby ionic conductivity therein is reduced, resulting in the degraded cycle performance.
Further, in the first non-aqueous electrolyte secondary battery, it is preferable to use a lithium-containing composite nickel oxide represented by LiNi1xe2x88x92XMXO2 (wherein M denotes at least one type of element selected from the group consisting of transition metals, B, Al, Si, and P, and the relationship, 0xe2x89xa6xxe2x89xa60.5, is satisfied) as the positive electrode material in order to prevent the overdischarge in the non-aqueous electrolyte secondary battery, thereby improving cycle performance. Particularly, in order to further prevent the overdischarge, it is preferable that the above-mentioned M is at least one type of element selected from the group consisting of Co, Ti, V, Mn, Fe, Sn, B, Al, Si, and P.
Examples of a lithium-containing composite nickel oxide used as the positive electrode material include LiNiO2, LiNi0.8Co0.2O2, LiNi0.8Al0.2O2, LiNi0.8Ti0.2O2, LiNi0.8V0.2O2, LiNi0.8Cr0.2O2, LiNi0.8Nn0.2O2, LiNi0.8Fe0.2O2, LiNi0.8Cu0.2O2, LiNi0.8Zn0.2O2, LiNi0.8Nb0.2O2, LiNi0.8Mo0.2O2, LiNi0.8Sn0.2O2, LiNi0.8W0.2O2, LiNi0.7Co0.1Ti0.2O2, LiNi0.8Mn0.1Al0.1O2, and the like.
On the other hand, examples of a lithium-containing titanium oxide used as the negative electrode material include Li4Ti5O12, Li3Ti3O8, and the like.
As a cyclic carbonic ester used as the solvent of the non-aqueous electrolyte solution, ethylene carbonate, propylene carbonate, butylene carbonate, and the like can be used. Among these, ethylene carbonate and propylene carbonate are particularly preferred. On the other hand, as a chain carbonic ester, dimethyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, methyl isopropyl carbonate, diethyl carbonate, ethyl propyl carbonate, ethyl isopropyl carbonate, and the like can be used. Among these, dimethyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, and diethyl carbonate are particularly preferred.
Solvents other than the above-mentioned cyclic carbonic ester and chain carbonic ester can also be added to the solvent of the non-aqueous electrolyte solution. Examples of such solvents include 1,2-diethoxyethane, 1,2-dimethoxyethane, and ethoxymethoxyethane and the like, which have been conventionally generally used in non-aqueous electrolyte secondary batteries.
When the total content of the above-mentioned cyclic carbonic ester and chain carbonic ester is not less than 80% by volume of the whole solvent, the side reaction that decrease the battery capacity is further prevented, whereby the cycle performance is further improved.
In the above-mentioned non-aqueous electrolyte solution, as a solute dissolved in the solvent as described above, a known solute which has been conventionally used in a non-aqueous electrolyte secondary battery can be used. Examples of such a solute include lithium compounds such as LiPF6, LiClO4, LiBF4, and LiCF3SO3. A non-aqueous electrolyte solution obtained by dissolving any one of the above-mentioned solutes in the above-mentioned solvent in the concentration of 0.5 to 1.5 mol/l is generally utilized.
Further, as a separator used to separate the positive electrode and the negative electrode in the first non-aqueous electrolyte secondary battery according to the present invention, a microporous film and unwoven fabric respectively made of polypropylene, polyethylene, or the like, which are conventionally generally utilized, can be used. It is also possible to use as a separator a solid electrolyte using polyethylene oxide, polyvinylidene fluoride, or the like, which is impregnated with the above-mentioned non-aqueous electrolyte solution.
A second non-aqueous electrolyte secondary battery according to the present invention is a non-aqueous electrolyte secondary battery provided with a positive electrode, a negative electrode, and a non-aqueous electrolyte solution, wherein a lithium-containing composite nickel oxide represented by LiNi1-xMnyMzO2 (wherein M denotes at least one type of element selected from the group consisting of Co, Ti, V, Fe, Sn, B, Al, Si, and P, and the relationships, x=y+z, xxe2x89xa60.6, and 0.05xe2x89xa6yxe2x89xa60.3, are satisfied) is used as a chief component of the positive electrode material for the positive electrode, a lithium-containing titanium oxide is used as a chief component of the negative electrode material for the negative electrode, and the solvent of the non-aqueous electrolyte solution contains a cyclic carbonic ester in an amount of not less than 10% by volume of the whole solvent.
As in the second non-aqueous electrolyte secondary battery according to the present invention, if the predetermined amount of Mn is contained in the lithium-containing composite nickel oxide and the solvent of the non-aqueous electrolyte solution contains a cyclic carbonic ester in an amount of not less than 10% by volume of the whole solvent, even when an amount of Ni contained in the lithium-containing composite nickel oxide is small, cycle performance is improved as in the case of the above-mentioned first non-aqueous electrolyte secondary battery. In addition, the positive electrode material can be obtained at lower cost as compared with that of the first non-aqueous electrolyte secondary battery.
In the second non-aqueous electrolyte secondary battery, ethylene carbonate, propylene carbonate, butylene carbonate, and the like also can be used as a cyclic carbonic ester used as the solvent of the non-aqueous electrolyte solution. Among these, ethylene carbonate and propylene carbonate are particularly preferred.
In using a cyclic carbonic ester as the solvent of the non-aqueous electrolyte solution, when the solvent of the non-aqueous electrolyte solution contains the cyclic carbonic ester in an amount of 30 to 70% by volume of the whole solvent, the side reaction that decrease the battery capacity is further prevented, whereby the cycle performance is further improved.